1. Field of the Invention
The present invention relates to synchronization of equipment items in an on-board communication network and more particularly to a method and a device for clock synchronization and time-stamping for equipment items of a communication network of AFDX or Ethernet type.
2. Discussion of the Background
In test, measurement or automation applications, it is frequently necessary to synchronize the local equipment item clocks precisely, so that they share a common time. By way of illustration, such a synchronization mechanism is employed in aircraft test and measurement applications aimed at measurements with strain gauges. It makes it possible to correlate the exchanged data with a time scale.
For these purposes, an equipment item provided with a precise clock, such as an equipment item provided with a GPS receiver (initials for Global Positioning System in English terminology) or connected to such a receiver, is generally responsible for distributing the time to all equipment items involved.
According to a common solution, a dedicated link, which can be precisely characterized in terms of transfer time, is used to assure routing of time-stamping messages. The IRIG protocol (acronym for Inter-Range Instrumentation Group in English terminology) 200-98 uses this principle, which is illustrated in FIG. 1.
In this case, on-board network 100 of AFDX type (initials for Avionics Full-Duplex Switched Ethernet in English terminology) comprises equipment items 105-1 to 105-6, which are interconnected by a switch 110. Since equipment item 105-1 in this case is connected to a GPS receiver 115, it is considered to be the reference equipment item in terms of synchronization. A time-stamping link 120 of “IRIG” type connects equipment item 105-1 to equipment items 105-2 to 105-6, independently of switch 110, or in other words independently of the AFDX links connecting switch 110 to equipment items 105-1 to 105-6.
Reference equipment item 105-1 has the task of distributing a reference clock signal, which originates from a GPS signal received by GPS receiver 115, via specific link 120 to all other equipment items (105-2 to 105-6), to permit synchronization thereof.
Although this solution is simple and effective, it nevertheless has disadvantages associated in particular with the necessity of employing a supplementary link, causing in particular costs for design, manufacture and maintenance.
Another solution consists in using IEEE Standard 1588, which permits precise synchronization and time-stamping via an Ethernet network. This standard is based on routing synchronization messages and time-stamping messages across the same communication network as that used to exchange data. According to this standard, the switches used modify the time-stamping messages to include the time of propagation of synchronization messages. The terminal equipment items collect these messages and determine the precise instant of their reception in order to correct their local clock correspondingly.
This solution assures good performances. However, it is complex to employ and it requires that the switches be capable of generating frames autonomously.
Thus, although solutions exist for synchronizing equipment items in a communication network of Ethernet or AFDX type and for permitting time-stamping of data and events, these solutions are not entirely satisfactory.